The present invention relates to a process for dehydrogenating organic compounds, in particular paraffins and naphthenes, to produce alkenes and aromatic compounds with optimised yields. The process of the invention is carried out in the presence of a bimetallic supported catalyst comprising a group VIII metal and at least one additional metal constituted by tin, at least a portion of which interacts strongly with said group VIII metal.
Aromatic compounds and alkenes constitute the feeds of choice for the petrochemicals industry.
Processes for dehydrogenating light paraffins can upgrade aliphatic hydrocarbons with a low boiling point, such as butanes and isobutanes, pentanes and isopentanes, that can be recovered after extracting the unsaturated compounds from steam cracked or catalytically cracked cuts. The process for dehydrogenating longer paraffins is an important commercial process because of the current demand for monoolefins to prepare biodegradable detergents or pharmaceutical products, for example.
While the principal sources of alkenes are catalytic cracking and steam cracking processes, these two processes also produce by-products and with increasing demand being oriented towards specific alkenes, it would not make economic sense to produce them by cracking.
For this reason, the direct production of alkenes remains in some cases an unavoidable step. This is the case for propylene, isobutene and linear long chain alkenes for the production of polypropylene, MTBE and LAB (linear alkyl benzene) respectively.
The principal features of the paraffin dehydrogenation reaction are because thermodynamic equilibrium limits the degree of conversion per pass and that the reaction is highly endothermic. These two characteristics determine the technological choice regarding the process and also the composition, structure and design of the catalyst.
High temperature operations are necessary to maintain a level of conversion close to thermodynamic equilibrium, but such high temperatures also encourage a certain number of side reactions leading to a lower quality product. Such reactions include reactions resulting in the formation of light products (cracking, hydrogenolysis), of highly unsaturated compounds that are precursors for carbonaceous deposits and thus initiators of deactivation (dehydrocyclisation, deep dehydrogenation) such as aromatic compounds or diolefins and skeletal isomerisation reactions responsible for the formation of branched molecules. Under those particularly severe operating conditions, it is very difficult to maintain high activity for long periods because of those secondary reactions.
Means for limiting those secondary reactions can be based on the process and/or on the catalytic formulation. To improve the performance of catalytic systems, in particular their stability, U.S. Pat. No. 4,716,143 describes a catalyst based on supported platinum wherein the distribution of the platinum is limited to the external surface of the support over a maximum depth of 400 xcexcm The advantage of such a choice resides in the fact that a distribution on the support periphery can limit side reactions and as a result, improve the performance of the catalyst. However, that type of distribution can only rarely produce homogeneous platinum/modifier atomic ratios on the particle scale (nanometer). Further, an over-concentration of active phase can cause diffusional limitations in the catalyst grain (extragranular diffusion) and thus reduce the overall reaction yield.
A vast number of patents and publications demonstrate that adding promoters to a base metal improves the performance of the catalyst. Such elements are added in different forms such as salts or organometallic compounds. In general, more active or more selective catalysts are obtained, which are sometimes more stable than the corresponding monometallic catalyst.
The formulation of catalysts used in hydrocarbon transformation processes, in particular catalysts for catalytic reforming and paraffin dehydrogenation, has been the subject of a large number of studies. Of the more frequently used promoters, tin can increase the selectivity and stability of the catalysts. Catalysts based on PtSn supported on alumina and used in paraffin dehydrogenation have, for example, been described in French patent FR-B-2 031 984 and U.S. Pat. No. 3,531,543.
In particular, catalysts based on PtSn contain different forms of tin. In the reduced state, those catalysts, supported on alumina, essentially contain species of tin in the oxidised state, namely species of divalent tin SnII and tetravalent tin SnIV, and minor quantities of tin in the reduced state Sn0 (M. C. Hobson et al., J. Catal., 142, 641-654 (1993), L. D. Sharma et al., Appl. Catal. A Genneral., 168, 251-259, (1998)). Those catalysts are generally prepared from a solution of tin chloride in an acidic medium (HCl, NHO3) and a hexachloroplatinic acid solution. The role of the tin present on the catalyst surface in oxidation state +2 or, more preferably, +4, is to minimise isomerisation and cracking reactions that occur at acidic sites on the support, and to limit coke formation to improve the stability of the catalyst.
One technique that can examine the local electronic structure of the tin (oxidation state, environment, chemical bonding) is Mxc3x6ssbauer spectroscopy, which directly provides two fundamental parameters: the isomer shift, xcex4 (IS) and the quadrupolar splitting xcex94 (QS). The isomer shift xcex4 measures the energy position of the Mxc3x6ssbauer absorption, a function of the density of the nucleus s, directly characterises the oxidation state of the tin. The quadrupolar splitting, xcex94, which defines the form of the absorption, is a function of the distribution of the surrounding charges, and characterizes the degree of coordination and thus the type of chemical bond in which the tin is involved. Each species of tin is characterized by a sub-spectrum defined by the two parameters IS and QS. Mxc3x6ssbauer spectroscopy also provides access to the line width LW, by comparison with the natural width of the emission (0.64 mm/s): the line width LW provides information regarding the degree of order and the distribution of the sites occupied by the tin. The relative intensity of the absorption for each species is proportional to the number of tin atoms and to the Mxc3x6ssbauer Lamb factor f, which represents the probability of resonant absorption without recoil and without thermal broadening. The factor f is directly related to the rigidity of the lattice and its value is increased by a reduction in the temperature of measurement. It can be small at ambient temperature (0.06 for the metallic xcex2 phase of tin) and thus requires measurements to be carried out at low temperatures. The proportion of each species is estimated from their contribution to the total absorption, provided that the recoil-free resonant absorption fractions f are not too different.
Characterisations using Mxc3x6ssbauer spectroscopy of reduced catalysts based on PtSn supported on alumina or silica mention the existence of a species Sn0 contained in a PtxSny type phase (x and y from 1 to 4) in which the tin is in oxidation state 0 (IS of 1.4 to 1.8 mm/s with respect to BaSnO3) in a form that is very close to bulk alloys characterized by a low or zero quadrupolar splitting (M. C. Hobson et al., J. Catal., 142, 641-654 (1993); Z. Huang et al., J. Catal., 159, 340-352 (1993); J. L. Margitfalvi et al., J. Catal., 190, 474-477 (2000); V. I. Kuznetov et al., J. Catal., 99, 159 (1986); R. Bacaud et al., J. Catal., 69, 399 (1981); R. Srinivasan et al., Catal. Today, 21, 83 (1994)). On alumina, the formation of metallic tin in the reduced state, favoured with larger metallic particle sizes of more than 2 nm, is responsible for the loss in performance of PtSn catalysts supported on alumina (Z. Huang et al., J. Catal., 159, 340-352, (1993), F. Yining et al., Stud. Surf. Sci. Catal., 68, 683-690, (1991)). A number of documents describe the use of catalysts containing a PtSn phase dispersed on alumina or tin that is essentially in a higher oxidation state than that of metallic tin (U.S. Pat. Nos. 3,846,283, 3,847,794). Under such conditions, the conventional preparation methods used cannot guarantee a close association between tin and platinum, an intimate association between those metals in the catalyst in the reduced state being generally desirable, however, to best exploit the bimetallic effect in processes for transforming organic compounds.
The invention concerns a process for dehydrogenating organic compounds carried out in the presence of a novel supported catalyst containing at least one metal from group VIII of the periodic table and at least tin at least a portion of which interacts strongly with said group VIII metal. The supported catalyst used in the process of the invention is characterized in that it contains metallic particles, of small size, less than 2 nm, and in that at least 10% of the tin species present in the catalyst in the partially re-oxidised state are in the form of a reduced tin species with oxidation state 0. Said reduced species is in a particular form, as demonstrated by 119Sn Mxc3x6ssbuaer spectroscopy, and is characterized by a very high quadrupolar splitting value of more than 0.65 mm/s and an isomer shift IS in the range 0.8 to 2.6 mm/s with respect to BaSnO3. This species is revealed by carrying out perfectly controlled oxidation on the reduced catalyst using pulses of oxygen. This particular species of tin is very closely associated with the group VIII metal and reveals a very strong interaction between the atoms of said group VIII metal and at least a fraction of the tin in the catalyst in the reduced state. As an example, in the case where the group VIII metal is platinum, a PtxSny phase is formed in which the tin has set values for IS and QS. Preferably, the catalyst also contains an alkali or alkaline-earth metal.
The catalyst used in the process of the invention has substantially improved catalytic properties with respect to prior art catalysts, in particular as regards activity and stability. It has surprisingly been discovered that the presence of a large quantity of a reduced tin species with oxidation state 0 and closely associated with a group VIII metal in a bimetallic catalyst that is partially oxidised by oxidation carried out under perfectly controlled conditions using pulses of oxygen, reveals a strong interaction in the reduced state of the catalyst between the group VIII metal and at least a fraction of the tin, guaranteeing a beneficial bimetallic effect on the catalytic performances of the units for dehydrogenating organic compounds, in terms of activity and stability, better activity and better stability very substantially increasing the alkenes and aromatics yield, the target products of paraffin and naphthene dehydrogenation reactions.
The process for dehydrogenating organic compounds of the invention comprises bringing a hydrocarbon feed into contact with a supported catalyst comprising at least one metal from group VIII of the periodic table and tin, at least a portion of which interacts strongly with the group VIII metal in the catalyst in the reduced state. In the remainder of the description, a distinction is made between the catalyst in the reduced state and the partially oxidised catalyst in that the quantity of tin reduced to oxidation state 0 and in intimate association with a group VIII metal is higher in the partially oxidised catalyst.
The invention concerns dehydrogenation of all types of organic compounds. More precisely, it concerns the dehydrogenation of short paraffins, i.e., saturated aliphatic organic compounds containing 2 to 5 carbon atoms, the dehydrogenation of long paraffins, i.e., saturated aliphatic organic compounds containing 6 to 22 carbon atoms, and the dehydrogenation of naphthenes, i.e., saturated cyclic organic compounds. Within the context of the invention, the naphthenes are preferably selected from the group formed by methylcyclohexane and cyclohexane.
The hydrocarbon feed brought into contact with the catalyst can comprise compounds other than those to be dehydrogenated; in particular, it can contain aromatic compounds.
The support for the catalyst used in the process of the invention comprises at least one refractory oxide that is generally selected from oxides of metals from groups IIA, IIIA, IIIB, IVA or IVB of the periodic table, such as oxides of magnesium, aluminium, silicon, niobium, titanium, zirconium and thorium, taken alone or as a mixture or mixed with oxides of other elements from the periodic table. For organic dehydrogenation reactions, the preferred support is alumina, with a specific surface area advantageously in the range 5 to 400 m2 per gram, preferably in the range 50 to 350 m2/g. The support for the catalyst of the invention can also be a zeolite or molecular sieve of type X, Y, mordenite, faujasite, ZSM-5, ZSM-4, ZSM-8, MFI, EUO, mazzite and mixtures of oxides of metals from groups IIA, IIIA, IIIB, IVA and IVB with the zeolitic material, in particular aluminium oxide-zeolite mixtures.
The group VIII metal is the catalytically active base metal of the catalyst used in the process of the invention. Preferably, it is a noble metal from the platinum family (Pt, Pd, Rh, Ir). More preferably, the noble metal is platinum. Advantageously, the catalyst contains a first noble metal (such as Pt) to which iridium is added. When dehydrogenating organic compounds, in particular paraffins, platinum and iridium are preferred. The percentage by weight is selected so as to be in the range 0.01% to 10%, preferably in the range 0.05% to 5%.
The tin acts as a promoter. The percentage by weight of tin in the catalyst in the reduced state, including all species (reduced and oxidised) with respect to the total catalyst weight, is in the range 0.01% to 2%, for example. Very advantageously, the catalyst used in the process of the invention contains at least 0.1% by weight of tin. In accordance with the invention, tin is present in the reduced catalyst essentially in the oxidised state (SnII and SnIV). An essential feature of the process of the invention is the use of a catalyst with a high proportion of metallic tin Sn0 with respect to the oxidised tin species, when the catalyst is in the partially oxidised state, i.e., when the reduced catalyst has undergone perfectly controlled oxidation using pulses of oxygen. This species of metallic tin Sn0 is in the very particular form of an VIII metal-Sn alloy, in which said group VIII metal and tin are intimately associated and strongly interact with oxygen. This species has very high QS values in the range 0.65 to 2.00 mm/s and is revealed when the catalyst is partially oxidised. The catalyst that is brought into contact with the hydrocarbon feed is characterized in that in the partially oxidised state, at least 10% of the tin with respect to the tin introduced is in the form of a reduced species with an oxidation state of 0, i.e., this reduced species with oxidation state 0 represents at least 10% of the tin present in the catalytic mass. Advantageously, said reduced tin species with oxidation state 0 represents at least 12% of the tin present in the catalytic mass. Preferably, it represents at least 15%, more preferably at least 20% and still more preferably at least 25%. Highly preferably, it represents at least 30%.
In the partially oxidised state of the catalyst, said reduced tin species generally does not represent more than 90% of the catalytic mass. Preferably, it does not represent more than 70% and more preferably, it does not represent more than 60%.
The term xe2x80x9ccatalyst in the partially oxidised statexe2x80x9d means a catalyst that has been oxidised in a perfectly controlled manner using pulses of oxygen. In accordance with the invention, perfectly controlled oxidation of the catalyst in the reduced state can reveal, by Mxc3x6ssbuaer spectroscopy, the presence of a large quantity of metallic tin in intimate association with the group VIII metal, in particular platinum, and oxygen, the presence of said reduced tin species resulting in the existence of a strong interaction between said group VIII metal, preferably platinum, and at least a fraction of the tin in the catalyst in the reduced state. The catalyst in its partially oxidised state contains Sn2+ species and said tin species reduced to oxidation state 0 (Sn0) is in the particular form of an VIII metal-Sn alloy, preferably in the particular form of a Ptxe2x80x94Sn alloy.
When the reduced catalyst is oxidised under non controlled conditions such as re-oxidation in air with a high partial pressure of oxygen, the amount of the metallic tin species Sn0 in the form of VIII metal-Sn alloy, preferably in the form of a PtSn alloy, reduces very substantially to the advantage of the formation of Sn4+ species. The catalyst is then strongly oxidised and essentially comprises Sn4+ species. It no longer contains Sn2+ species. This formation of Sn4xe2x88x92 species perturbs the quantification of Sn species strongly interacting with the group VIII metal, preferably platinum, present from the reduced state, and thus the performance of the catalyst in its reduced state cannot be appreciated. In contrast, controlled oxidation by pulsing can selectively form said reduced tin species Sn0 with the Mxc3x6ssbuaer characteristics described above, avoiding the formation of Sn4xe2x88x92 species.
The catalyst can also optionally contain 0.1% to 3% by weight of at least one alkali metal or alkaline-earth metal. Paraffin dehydrogenation is most preferably carried out with a catalyst comprising such an element (alkali or alkaline-earth). In order to dehydrogenate short paraffins, potassium is preferably used, and in order to dehydrogenate long paraffins, lithium is preferably used.
The catalyst can also contain at least one group IIIA element. The group IIIA element is selected from indium, gallium and thallium, preferably indium, in an amount in the range 0.005% to 3% with respect to the total catalyst weight, more preferably in the range 0.1% to 1%. Most advantageously, the catalyst used for dehydrogenating long paraffins contains indium. The catalyst can also optionally contain, for example, at least one halogen or a halogenated compound in proportions of the order of 0.1 to 3% by weight of catalyst. It can also optionally contain at least one metalloid such as sulphur in proportions of the order of 0.01% to 2% by weight of catalyst. It can also contain at least one other chemical element, for example rhenium or niobium, in proportions of the order of 0.01% to 3% by weight of catalyst, said element being introduced into the catalyst using any method and in any form that is known to the skilled person.
The catalyst can be in the form of beads, extrudates, trilobes or any other routinely used form. The catalyst of the invention contains metallic particles with small sizes, i.e., less than 2 nm. These metallic particles contain the noble metal and tin, the tin being mainly in oxidation state 0 in proportions as described above.
Analyses that can examine the local electronic structure of the tin are carried out in a conventional Mxc3x6ssbuaer spectroscopy provided with a Ba119mSnO3 xcex3 radiation source with a nominal activity of 10 mCi. The spectrometer operates in transmission mode with a velocity transducer with a constant acceleration functioning in triangular mode with respect to a 512-channel multichannel analyser, controlled by a microcomputer. The detector is a 0.1 mm thick NaI (T1) crystal scintillator. The scale is calibrated using a standard 6-line xcex1-Fe spectrum obtained with a 57Co(Rh) source. All of the isomer shifts IS are given with respect to a BaSnO3 standard. ISO software is used to resolve the experimental spectra into Lorentzian profiles and into the different parameters, plus the mean squares (W. Kxc3xcnding, Nucl. Instrum. Method., 75, 336 (1969)).
For certain analyses carried out at low temperature, a cryostat with a variable flow and temperature (4 to 300 K) is advantageous. Such measurements are necessary to characterize values of f relating to a given species.
The analyses are carried out using powdered catalysts, already reduced, in a hydrogen flow, between 450xc2x0 C. and 550xc2x0 C. After cooling to ambient temperature in hydrogen and flushing with a neutral gas such as helium, the reduced catalyst undergoes the number of pulses of oxygen necessary to saturate it. Pulsed injections are continued until at least 10 peaks are obtained with a constant surface area (chromatographic analysis), then the catalyst is flushed with a neutral gasoline such as helium, the treatment cell is sealed directly without ingress of air. The quantity of catalyst necessary, including the amount of tin, is at least 2 g. This cell can be used at ambient temperature or at low temperatures. The catalyst tested using Mxc3x6ssbuaer spectroscopy is in the partially re-oxidised state.
The catalyst used in the process of the invention and analysed by Mxc3x6ssbuaer spectroscopy in the partially oxidised form, contains tin in the oxidised form (divalent or tetravalent tin) and in the reduced form. In accordance with the invention, the SnIV species are characterized by an isomer shift IS in the range 0 to 0.25 mm/s and with a quadrupolar splitting QS in the range 0 to 0.80 mm/s. SnII species are characterized by an IS in the range 2.70 to 3.40 mm/s and a QS in the range 1.60 to 2.60 mm/s. Sn0 species are characterized by an IS in the range 0.80 to 2.60 mm/s and a QS in the range 0.65 to 2.00 mm/s.
In accordance with the invention, said reduced tin species in oxidation state 0 (Sn0) is a particular form of the VIII metal-Sn alloy, preferably in the particular form of a Ptxe2x80x94Sn alloy, with values of IS in the range 0.80 to 2.60 mm/s, preferably in the range 0.80 to 1.50 mm/s, more preferably in the range 1.10 to 1.40 mm/s and with values of QS in the range 0.65 to 2.00 mm/s, preferably in the range 0.80 to 2.00 mm/s, more preferably in the range 0.90 to 1.90 mm/s and still more preferably in the range 0.95 to 1.50 mm/s. Advantageously, said reduced tin species in oxidation state 0 (Sn0), in the particular form of a VIII metal-Sn alloy, and in particular in the form of a PtSn alloy, and with values of IS and QS as given above, is only present in the catalyst in the partially oxidised state.
The values for the Mxc3x6ssbuaer parameters obtained for the catalyst in the partially oxidised state are the results of the existence of a strong interaction between the tin and the group VIII metal, this interaction being revealed by perfectly controlled oxidation of the reduced catalyst.
The very close association of these two metals in the catalyst of the invention can exploit to the limit the intrinsic properties of each metal and thus generates a synergistic effect that is even greater when the reduced tin species with oxidation state 0 is in a large quantity and has a high QS. As an example, when the group VIII metal is platinum, the reduced tin species is contained in a particular phase of the type PtxSny where said reduced tin species and the platinum are in intimate association.
The tin precursor can be selected from the group formed by halogenated compounds, hydroxides, oxides, carbonates, carboxylates, nitrates and sulphates of tin, this list being non-limiting. It can be introduced in the form of at least one organic compound selected from the group formed by tin complexes, and hydrocarbyl tin compounds such as tin alkyls, cycloalkyls, aryls, alkylaryls and arylalkyls. The tin precursor can also be selected from the group formed by halogenated compounds, hydroxides, oxides, carbonates, carboxylates, nitrates and sulphates of organometallic tin compounds, this list being non-limiting. These compounds comprise at least one carbon-Sn bond. As an example, the tin precursor can be selected from polyalkyl halides, for example trimethyl halides (Me3SnX), triethyl halides (Et3SnX), dimethyl dihalides (Me2SnX2), diethyl dihalides (Et2SnX2), diisopropyl dihalides (iPr2SnX2), di-n-propyl dihalides (n-Pr2SnX2), methyl trihalides (MeSnX3), ethyl trihalides (EtSnX3), isopropyl trihalides (i-PrSnX3), di-n-propyl trihalides (n-PrSnX3), polyalkyl hydroxides, for example trimethyl hydroxides (Me3SnOH), triethyl hydroxides (Et3SnOH), dimethyl dihydroxides (Me2Sn(OH)2), diethyl dihydroxides (Et2Sn(OH)2), diisopropyl dihydroxides (iPr2Sn(OH)2), n-propyl dihydroxides (n-Pr2Sn(OH)2), methyl trihydroxides (MeSn(OH)3), ethyl trihydroxides (EtSn(OH)3), diisopropyl trihydroxides (iPrSn(OH)3), n-propyl trihydroxides (n-PrSn(OH)3), polyalkyl acetates, for example trimethyl acetate (Me3SnOC(O)Me), triethyl acetate (Et3SnOC(O)Me), tributyl acetate (Bu3SnOC(O)Me), polyalkyl oxides, for example bis-trimethyl oxide ([Me3Sn]2O), bis-triethyl oxide ([Et3Sn]2O), bis-tripropyl oxide ([Pr3Sn]2O bis-tributyl oxide ([Bu3Sn]2O), polyalkyl sulphates, for example bis-trimethyl sulphate ([Me3Sn]2SO4), bis-dimethyl sulphate ([Me2Sn]2SO4), methyl trioxo sulphate (MeSnO3), where X represents a halogen selected from the group formed by fluorine, chlorine, bromine and iodine. The tin precursor can be selected from compounds with general formula (R1)xM(R2)y(R3)z, where x+y+z=the valency of tin and where R1is selected from the group formed by alkyl, cycloalkyl, nitrile (CN), carbonyl (CO), aryl, alkylaryl and arylalkyl radicals, where R2 is a function with the form CaHbRxe2x80x2c, where Rxe2x80x2 represents a hydroxide, halide, carboxylate, PO3H or SO3H function and where R3 is an aquo, oxo (MO), alkoxide (O-alkyl), hydride, hydroxyl, alkylsulphonate, alkylsulphate, thioalkyl, N(SO3Rxe2x80x3)2, PRxe2x80x32 and PRxe2x80x33 group, where Rxe2x80x3 is an alkyl group (xe2x80x9cHandbook of physics and chemistryxe2x80x9d, 63rd edition, 1982-83).
The terms xe2x80x9calkyl groupsxe2x80x9d means groups comprising linear, branched or cyclic saturated carbon atoms and hydrogen atoms. The term xe2x80x9caryl groupsxe2x80x9d means aromatic groups.
At least one alkyl group in the compounds cited above can be replaced by an alkenyl group, i.e., a group comprising linear, branched or cyclic unsaturated carbon atoms and hydrogen, for example an allyl group.
Preferred tin precursors are organometallic compounds of the type SnR4 (R=alkyl group) or polyalkyl halides such as Me3SnCl, Me2SnCl2, MeSnCl3, Et3SnCl, Et2SnCl2, EtSnCl3, iPrSnCl2 and the hydroxides Me3SnOH, Me2Sn(OH)2, Et3SnOH, Et2Sn(OH)2, the oxide [Bu3Sn]2O, or the acetate Bu3SnOC(O)Me. These polyalkyl halides comprise at least one carbon-Sn bond and at least one hydrosoluble function, which renders them soluble in aqueous solvents, facilitating processing when preparing the catalyst.
The group VIII metal compound can be introduced in the form of an inorganic or organic complex selected, for example, when the group VIII metal is platinum, from hexachloroplatinic acid, hexahydroxyplatinic acid, dihydroxytetramine platinum, platinum diaminonitrite, or from organometallic complexes such as platinum bis-acetylacetonate.
Preparation of the catalyst includes simultaneous or successive introduction in any order of the group VIII metal, tin, optional halogen or halogenated compound, optional alkali or alkaline-earth metal, optional metalloid, and optional other chemical element. When the elements are introduced successively, once the first element has been introduced, the skilled person will then be able to adapt the conditions for introducing the other elements so as to obtain a catalyst with the characteristics defined above.
The metals can be introduced during any of the catalyst manufacturing steps using prior art techniques. As an example, the tin can be added to an alumina sol (U.S. Pat. No. 3,929,683) or when the support is being formed, for example using extrusion forming (U.S. Pat. No. 3,917,808) or by the oil drop method (U.S. Pat. No. 3,558,508). In a preferred implementation of the process for preparing the catalyst, the support is preferably initially impregnated using a solution of an alkali or alkaline-earth metal, the mixture is filtered then the recovered product is dried and calcined. The solid obtained is impregnated with an aqueous solution saturated with CO2, containing at least one tin precursor in the form of SnCl2 or, preferably, in the form of organometallic compounds containing at least one carbon-tin bond such as polyalkyl halides, for example Me3SnCl, Me2SnCl2, MeSnCl3, Et3SnCl, Et2SnCl2, EtSnCl3, iPrSnCl2 and the hydroxides Me3SnCl, Me2Sn(OH)2, Et3SnOH, Et2Sn(OH)2, the oxide [Bu3Sn]2O, or the acetate Bu3SnOC(O)Me. After leaving the solid and impregnating solution in contact for several hours, the product is filtered then optionally undergoes a drying step at 120xc2x0 C. and an optional calcining step between 300xc2x0 C. and 600xc2x0 C., preferably between 450xc2x0 C. and 550xc2x0 C. The solid obtained is preferably impregnated-with an organic solution of at least one compound of a group VIII metal, the volume of the solution being in excess with respect to the retention volume of the support. After several hours contact, the product obtained is dried and calcined in air between 300xc2x0 C. and 600xc2x0 C., preferably in a stream of air for several hours.
In a further, preferred, implementation of the process for preparing the catalyst, the catalytically active base metal such as platinum is deposited in a plurality of steps before depositing the tin, to selectively deposit the tin on particles with a controlled size, i.e., on particles with a size that is larger than that of the final catalyst. As an example, the support is impregnated with an organic solution containing at least one organometallic platinum compound such as platinum acetylacetonate (Pt(acac)2), the volume of the solution preferably being in excess with respect to the retention volume of the support. After leaving the solid and impregnating solution in contact for several hours, the product is filtered then dried and calcined in air between 300xc2x0 C. and 600xc2x0 C., preferably between 400xc2x0 C. and 500xc2x0 C., advantageously flus with air for several hours. It is then reduced in a stream of hydrogen between 300xc2x0 C. and 600xc2x0 C., preferably between 350xc2x0 C. and 500xc2x0 C. The catalyst is then transferred to the impregnation reactor without ingress of air, to deposit platinum again following exactly the same procedure as that given above. This can be carried out a number of times. To deposit the tin, the solid obtained is transferred to a reactor without ingress of air where tin impregnation is carried out by bringing an aqueous or organic solution of an organometallic tin compounds into contact for several hours, the volume of the solution preferably being in excess with respect to the retention volume of the support. The reaction is advantageously carried out in a stream of hydrogen into the impregnating solution. The solid obtained is filtered, dried and reduced in a stream of hydrogen between 300xc2x0 C. and 600xc2x0 C. If the catalyst contains an alkali or alkaline-earth metal, it can be added at any stage during the preparation. Preferably, it is introduced once tin impregnation has been carried out. Finally, the catalyst is calcined.
To dehydrogenate short paraffins, regardless of the process by which it is prepared, the catalyst advantageously undergoes a final step consisting of an oxychlorination treatment, generally carried out in a stream of a gas comprising oxygen, chlorine and possibly water using any technique that is known to the skilled person (U.S. Pat. No. 3,875,049).
In contrast, when dehydrogenating long chain paraffins, it is not advisable to carry out an oxychlorination treatment on the catalyst, as the presence of chlorine reduces the performances in such processes. Further, it is preferable to use precursors of metals comprised in the catalyst which may not contain chlorine. As an example the platinum precursor may be platinum acetylacetonate, and the tin precursor may be Me3SnCl or Me3SnOH.
Before use, and regardless of the type of compounds to be dehydrogenated, the catalyst is reduced in hydrogen, for example between 200xc2x0 C. and 600xc2x0 C., to obtain an active metallic phase. The procedure for this treatment consists, for example, in slowly raising the temperature in a stream of hydrogen to the maximum reduction temperature, for example in the range 200xc2x0 C. to 600xc2x0 C., preferably in the range 250xc2x0 C. to 550xc2x0 C., more preferably in the range 350xc2x0 C. to 550xc2x0 C., followed by a constant temperature stage for 1 to 6 hours at that temperature.
This reduction can be carried out immediately after calcining or subsequently on site. It is also possible to directly reduce the dried product on site.
Any other preparation method is suitable that results in a reduced catalyst with a strong interaction between at least a fraction of the tin and a group VIII metal and in the partially oxidised form containing at least 10% of the tin in the form of tin with an oxidation state of 0, where the reduced tin species Sn0 has an isomer shift in the range 0.80 to 2.60 mm/s and a quadrupolar splitting in the range 0.65 to 2.00 mm/s.
When the catalyst of the process of the present invention contains sulphur, the sulphur is introduced into the formed, calcined catalyst containing the metal or metals cited above, either in situ prior to the catalytic reaction, or ex situ. Optional sulphurisation is carried out after reduction. With in situ sulphurisation, if the catalyst has not already been reduced, reduction takes place before sulphurisation. With ex situ sulphurisation, reduction is carried out followed by sulphurisation. Sulphurisation is carried out in the presence of hydrogen using any sulphurisation agent that is well known to the skilled person, such as dimethyl sulphide or hydrogen sulphide. As an example, the catalyst is treated with a feed containing dimethyl sulphide in the presence of hydrogen, with a concentration such that the sulphur/metal atomic ratio is 1.5. The catalyst is then kept at about 400xc2x0 C. for about 3 hours in the stream of hydrogen before injecting the feed.
The different paraffin and naphthene dehydrogenation processes are distinguished by the choice of operating conditions and the composition of the feed. The operating conditions are adjusted depending on the feed to be treated, to obtain the best pressure-temperature yield and activity balance in a manner that is known to the skilled person.
The paraffin dehydrogenation reaction is generally carried out at a pressure in the range 0.02 to 2 MPa, preferably in the range 0.1 to 1 MPa, and at a temperature in the range 400xc2x0 C. to 800xc2x0 C. depending on the nature of the feed. The temperature is advantageously in the range 400xc2x0 C. to 550xc2x0 C. for a feed essentially comprising isopentane. The temperature is advantageously in the range 450xc2x0 C. to 550xc2x0 C. for a feed principally comprising paraffins containing 9 to 22 carbon atoms per molecule. The feed can also contain unsaturated hydrocarbons containing 3 to 22 carbon atoms per molecule. The mass flow rate of the feed treated per unit mass of catalyst is generally in the range 0.5 to 100 kg/kg/h. It may be advantageous to use hydrogen as the diluent. The hydrogen/hydrocarbon mole ratio is generally in the range 0 to 20, preferably in the range 0 to 6.
The naphthene dehydrogenation reaction is generally carried out at a pressure in the range 0.1 to 2 MPa, preferably in the range 0.1 to 1 MPa and at a temperature in the range 200xc2x0 C. to 400xc2x0 C. depending on the nature of the feed. The mass flow rate of the feed treated per unit mass of catalyst is generally in the range 0.5 to 100 kg/kg/h.
The following examples illustrate the invention without limiting its scope.